1. Field of the Invention
The present invention relates to an image correction processing method and apparatus for correcting a pixel value of each pixel constituting image data obtained from an original image affected by a so-called “peripheral light-off phenomenon”. The invention relates also to a program for implementing such method executable by a computer as well as to a program storage medium storing the computer-executable program. In particular, the invention relates to an art for correcting a pixel value of each pixel of image data obtained from an original image when the original image recorded on a photographic film is read as digital image data and then a print paper is exposed according to the data by a digital exposure technique.
2. Description of the Related Art
Conventionally, a photographic printing operation has been done by the analog exposure (projection exposure) technique wherein light is irradiated on a photographic film recording an original image and the light transmitted through the image-bearing film is irradiated on a print paper for obtaining a print of the image. In recent years, the digital exposure technique has been employed. In this case, an image recorded on a photographic film is read by e.g. a scanner to obtain digital image and then for each pixel, respective single color components of red, blue and green are irradiated on the print paper for obtaining a print.
On the other hand, in the case of a photography made by using e.g. a disposable camera (“one-time-use camera”), the image recorded on a film by such camera sometimes suffers density irregularity with its brightness progressively decreasing in the direction away from the center of the image. This phenomenon, referred to as the “peripheral light-off”, results from light amount irregularity due to aberration of the lens of the camera and/or insufficient light amount of a flash bulb of the camera used in a flash photography.
The light amount irregularity due to the lens aberration of a camera can occur in a following situation for instance. In general, when a photography is taken with an open aperture, the resultant photographed image will be significantly influenced by the lens aberration. In this case, in the image area on the film, the light amount gradually decreases in the direction away from the center of the image. Especially, in the case of such inexpensive cameras as the disposable cameras noted above, they often employ a lens having a relatively large aberration and they usually lack any aperture adjusting mechanism also. Therefore, with such cameras, the problem of light amount reduction in the peripheral areas of the image (peripheral light-off) tends to appear more conspicuously.
Needless to say, if an image recorded on a film with the influence of peripheral light-off is printed as it is on a print paper by the digital exposure or analog exposure, the resultant print also will be affected by the peripheral light-off, so that a quality print cannot be obtained.
In an attempt to cope with the above-described problem of image quality deterioration due to peripheral light-off phenomenon, the convention has sometimes implemented a technique called “mask printing”. This mask printing is a technique wherein during the exposure a portion of a film image is covered with a mask for varying an exposure time according to respective areas of the image. More particularly, if such reduction of light amount is present in the peripheral areas of the image, there is provided a period when the exposure is effected with the central area of the image being covered with the mask so that the exposure time of the central area may be shorter than that of the peripheral areas.
With such mask printing technique, however, there inevitably occurs discontinuity in the exposure amount in the border area in the image between the masked area and the non-masked area. On the other hand, the reduction of light amount in the peripheral areas (peripheral light-off) basically occurs in a continuous manner. Hence, a printed image obtained by this technique will appear “unnatural”.
In order to avoid such discontinuity in the exposure amount, the convention has also proposed to employ a special filter adapted for providing continuously varying transmission light amount as the mask to be employed in the mask printing technique. That is, for such film image bearing the influence of peripheral light amount reduction, for exposure of this film, there is employed a filter adapted for providing a small transmission light amount to the central area of the image while providing a progressively large transmission light amount to the peripheral areas thereof away from the central area. With this modified mask printing technique, the discontinuity of exposure amount can be avoided, so that a reasonably good quality of print can be obtained.
However, the reduction of peripheral light amount differs in degree for each of a plurality of images recorded on a film. Therefore, for appropriate correction of the peripheral light amount reduction, it is necessary to prepare a great number and many kinds of such filters for selective use. And, the selection of a best suited filter for each image is difficult and troublesome. And, the preparation of many filters invites disadvantageous cost increase. Moreover, even if such great number and kinds of filters are made ready for use, it is still difficult to achieve perfect match between one particular filter and a particular degree of peripheral light amount reduction present in each image.
In the case of the digital exposure, to cope with the same problem as above, the convention has proposed a technique in which when an image on a photographic film is read by a scanner or the like, a correction is made, if instructed by an operator, for increasing pixel values of pixels of the peripheral areas of the image and then this corrected image data is transmitted to an image processing unit for subsequent processing.
The above technique, however, depends simply on the selection between effecting the correction (for increasing the pixel values for the peripheral areas of the image) and not effecting it. Hence, this technique fails to provide appropriate correction according to each particular degree of peripheral light amount reduction present in each image. For certain types of images, the technique can hardly provide any correction effect to them at all.
Moreover, the correction made on the read data does not necessarily cope with each particular degree of peripheral light amount reduction in each image, that is, variation in the peripheral light amount. Therefore, such correction technique may even render the image data “unnatural” rather than “more natural”.